4 #include <linux/rcupdate.h>
17 * A struct pid is the kernel's internal notion of a process identifier.
18 * It refers to individual tasks, process groups, and sessions. While
19 * there are processes attached to it the struct pid lives in a hash
20 * table, so it and then the processes that it refers to can be found
21 * quickly from the numeric pid value. The attached processes may be
22 * quickly accessed by following pointers from struct pid.
24 * Storing pid_t values in the kernel and refering to them later has a
25 * problem. The process originally with that pid may have exited and the
26 * pid allocator wrapped, and another process could have come along
27 * and been assigned that pid.
29 * Referring to user space processes by holding a reference to struct
30 * task_struct has a problem. When the user space process exits
31 * the now useless task_struct is still kept. A task_struct plus a
32 * stack consumes around 10K of low kernel memory. More precisely
33 * this is THREAD_SIZE + sizeof(struct task_struct). By comparison
34 * a struct pid is about 64 bytes.
36 * Holding a reference to struct pid solves both of these problems.
37 * It is small so holding a reference does not consume a lot of
38 * resources, and since a new struct pid is allocated when the numeric pid
39 * value is reused (when pids wrap around) we don't mistakenly refer to new
45 * struct upid is used to get the id of the struct pid, as it is
46 * seen in particular namespace. Later the struct pid is found with
47 * find_pid_ns() using the int nr and struct pid_namespace *ns.
51 /* Try to keep pid_chain in the same cacheline as nr for find_pid */
53 struct pid_namespace *ns;
54 struct hlist_node pid_chain;
60 /* Try to keep pid_chain in the same cacheline as nr for find_pid */
62 struct hlist_node pid_chain;
63 /* lists of tasks that use this pid */
64 struct hlist_head tasks[PIDTYPE_MAX];
67 struct upid numbers[1];
70 extern struct pid init_struct_pid;
74 struct hlist_node node;
78 static inline struct pid *get_pid(struct pid *pid)
81 atomic_inc(&pid->count);
85 extern void FASTCALL(put_pid(struct pid *pid));
86 extern struct task_struct *FASTCALL(pid_task(struct pid *pid, enum pid_type));
87 extern struct task_struct *FASTCALL(get_pid_task(struct pid *pid,
90 extern struct pid *get_task_pid(struct task_struct *task, enum pid_type type);
93 * attach_pid() and detach_pid() must be called with the tasklist_lock
96 extern int FASTCALL(attach_pid(struct task_struct *task,
97 enum pid_type type, struct pid *pid));
98 extern void FASTCALL(detach_pid(struct task_struct *task, enum pid_type));
99 extern void FASTCALL(transfer_pid(struct task_struct *old,
100 struct task_struct *new, enum pid_type));
103 * look up a PID in the hash table. Must be called with the tasklist_lock
104 * or rcu_read_lock() held.
106 extern struct pid *FASTCALL(find_pid(int nr));
109 * Lookup a PID in the hash table, and return with it's count elevated.
111 extern struct pid *find_get_pid(int nr);
112 extern struct pid *find_ge_pid(int nr);
114 extern struct pid *alloc_pid(void);
115 extern void FASTCALL(free_pid(struct pid *pid));
117 static inline pid_t pid_nr(struct pid *pid)
125 #define do_each_pid_task(pid, type, task) \
127 struct hlist_node *pos___; \
129 hlist_for_each_entry_rcu((task), pos___, \
130 &pid->tasks[type], pids[type].node) {
132 #define while_each_pid_task(pid, type, task) \
136 #endif /* _LINUX_PID_H */